Technical Field
The invention relates generally to linear actuators and, in particular, to voice coil motor operated linear actuators.
Background Information
A laser beam material processing system requires a laser beam to remain in focus with respect to the surface of an object that is being scribed, engraved or drilled. For a planar work surface this is referred to as field flattening. The dynamic requirements of field flattening are rather modest and require relatively lower performance with respect to the x and y mirror speeds scanner. If the object is non-planar, transitions in the surface require dramatically increased dynamic adjustment of the focal plane of the scanned laser beam up or down in order that the beam remain in focus at the surface. As the laser beam is scanned across the non-planar object, a small pre-objective lens or lens group situated in the optical path moves along the beam axis causing the height at which the beam is in focus, i.e., the focal plane, to move up or down. The pre-objective lens or lenses must thus rapidly move over a range of several millimeters with extremely high acceleration to ensure that the focal point of the laser beam remains at the surface. Examples of such systems are a laser scribing system and a high dynamic focus adjuster used in a pre-objective scanner or coupled to a traditional F-theta style laser scanning system to enhance deep engraving and hole drilling capability.
In order to remain in focus as the beam is scanned across an object with sharp vertical transitions, for example, steps in height, the pre-objective lens needs to essentially instantaneously reposition. Accordingly, there is a great incentive to optimize the lens movement to achieve as high rate of acceleration as possible so that the dynamic focus of the laser beam can follow the sharp vertical transitions. Further, the fast dynamic focus enables both faster field flattening and high scan rates over the non-flat contoured or stepped surfaces, and thus, reduces process cycle time to scribe, engrave or drill the surface.
The rapid translational movement of the lens, and in some conditions rapid small amplitude movements, puts stress on associated bearings that promote the movement. Ball bearings, for example, are particularly susceptible to early wear when operating at rapid, small amplitude oscillations, due to the propensity to displace lubricant between the balls and races when the motion covers only a fraction of a ball rotation. Accordingly, the system may require additional maintenance and suffer associated down time.
Further, additional complexity is required to translate the rotational movement provided by the ball bearings to the desired translational movement of the lens along the beam axis.